Composite material of continuous fiber and ultra high molecular weight polyethylene

ABSTRACT

A composite material composed of continuous high strength fibers such as carbon, aramid or glass, and ultra high molecular weight polyethylene (UHMW PE), wherein the UHMW PE comprises a continuous matrix among and surrounding the fibers. The resultant composite material exhibits extraordinarily high strength, stiffness and other fiber dominated properties in the directions parallel to the fibers and exhibits its lowest strength and stiffness in directions perpendicular to the fibers. It also exhibits superb abrasion resistance, good impact strength, excellent chemical resistance, a low coefficient of friction and other beneficial properties of UHMW PE.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 from provisionalapplication Ser. No. 60/578,420, filed Jun. 9, 2004, and entitled “AComposite Material of Continuous Fiber and Ultra High Molecular WeightPolyethylene”.

FIELD OF THE INVENTION

This invention relates generally to the art of composite materials, moreparticularly to composite materials comprising thermoplastics and highstrength fibers.

BACKGROUND OF THE INVENTION

Thermoplastics are plastics that are made by heating resin pellets orpowders (the raw material) until they melt, molding the material to adesired shape, and then resolidifying the material through a coolingprocess. Because the chemistry of the plastic does not change, thisprocess can be performed multiple times. In other words, thermoplasticscan be reprocessed or recycled with additional heating and cooling.Additionally, the manufacturing process for thermoplastic products canbe as quick as the heating, molding, and cooling processes can occur(e.g. seconds). Hence thermoplastics have the manufacturing advantagesof rapid processing and multi-step manufacturing, both of which cantranslate and have translated to significant benefits and cost savings.

Some common thermoplastics are polyethylene “PE”, polypropylene “PP”,polyamide “PA” (more commonly known as nylon) and polyethyleneterepthalate “PET” (more commonly known as polyester). Products madefrom such thermoplastics are very diverse, for example, toys, rope,clothing and bottles. Melt temperatures for most thermoplastics fallbetween 125° C. and 300° C. In their softened or partially melted statethermoplastics flow under pressure and thus can be shaped by processessuch as injection molding, blow molding and extrusion.

Thermoplastics have many excellent qualities. Metals, however, generallyhave much higher strength and stiffness than thermoplastics. To increasethe strength and/or stiffness of thermoplastics it has long been knownthat mixing thermoplastic resins with short high strength fibers resultsin a product with substantially increased strength and/or stiffness.Often higher temperature properties are also improved. This mixing isdone either before melting the thermoplastic resin or as thethermoplastic resin is melted but in either case before forming theproduct by extrusion or injection molding, as disclosed for example inU.S. Pat. No. 3,577,378. Types of fiber commonly used for this purposemay be glass, carbon, aramid or other materials which have suitably highstrength and/or stiffness and melt temperatures above the thermoplasticof choice.

These materials are sometimes referred to as short fiber composites ormore commonly fiber reinforced plastics “FRP”. Such FRP materialstypically comprise 10 to 30 percent fiber by weight with the balancebeing the thermoplastic of choice. The fibers are typically 0.1 to 5 mmin length. The use of higher percentages of fiber and/or longer fibersis typically avoided because of difficulties achieving good flow of themixture and good uniform dispersion and distribution of the fibers,unacceptable accumulation of the fibers in the nozzle of the injectionmolding equipment, and unacceptable reduction in toughness and impactstrength in the product.

Typically with the short fibers from 0.1 to 5 mm in length, FRPthermoplastics have tensile and/or flexural strength and stiffness up to5 times greater than the same thermoplastic without the reinforcingfibers.

U.S. Pat. No. 3,577,378 teaches that the added tensile and flexuralstrength and stiffness is due to the high strength and stiffness of thefibers coupled with the excellent mixing and distribution of the fibersin the polymer and the ease with which the molten FRP material flows inthe extruder and injection mold. There is possibly, however, acharacteristic loss of impact strength in the short fiber FRP productcompared with the unreinforced thermoplastic product as reported withrespect to polycarbonate in U.S. Pat. No. 3,577,378.

A more current example of the enhanced strength and stiffness propertiesof FRP comes from Victrex plc, headquartered at Hillhouse Internationalin Lancashire, UK. Victrex is the world's largest producer of certainhigh performance high temperature engineered thermoplastics, inparticular polyetheretherketone (PEEK). Published Victrex data forunreinforced “150G” PEEK and 30% short carbon fiber reinforced “150CA30”PEEK is indicative of the difference in properties that successfulintroduction of short fibers can produce, as illustrated in FIG. 1. Thisdata is excerpted from Victrex USA Inc. publication 1100/2.5m titledPEEK Material Properties Guide. Both materials are suitable forextrusion and injection molding.

Engineers and designers, however, continue to seek materials which havestrength and stiffness that exceed conventional FRP short fiberthermoplastics and that also have strength to weight and stiffness toweight ratios that are higher than those of metals. In the past twentyfive years several processes have been invented wherein thermoplasticcomposites have been produced with continuous fibers and with fibercontent as high as 70%. These materials generally have much higherstrength and/or stiffness than short fiber FRP materials. In fact thesecomposites exhibit strength and/or stiffness comparable and in manycases superior to metals, including steel. Also, generally, the densityof these materials is much lower than metals. Hence their strength toweight and stiffness to weight ratios are much higher than metals,including steel. U.S. Pat. No. 4,680,224 discloses a preferred methodfor producing such continuous fiber thermoplastic composites.

This APC-2 thermoplastic composite manufactured by Cytec EngineeringMaterials, a division of Cytec, Inc. It is made with continuousunidirectional AS4 carbon fiber and PEEK thermoplastic resin. Forillustration of the enhanced strength of these continuous fibercomposite thermoplastics, FIG. 2 depicts selected mechanical propertiesat room temperature of APC-2. The fiber content in this composite is 68%by weight. Note the enhanced properties in the fiber direction ascompared to the unreinforced PEEK and the FRP short fiber PEEKproperties listed in FIG. 2. The strength in the fiber direction is 9times greater than the 150CA30 short fiber PEEK FRP and more than 20time greater than the unreinforced 150 PEEK. The strength and stiffnessperpendicular to the fibers is however, much lower. The tensile strengthperpendicular to the fibers is approximately the same as theunreinforced resin while the stiffness perpendicular to the fibers isabout double the unreinforced resin.

These values clearly demonstrate both the tremendous strength andstiffness of continuous fiber thermoplastic composites and the influenceof fiber direction at the same time. This directional or non-isotropicnature of continuous fiber composites allows the designer much greaterfreedom to create optimized designs when compared to most conventionalplastics and metals which have more or less isotropic or non directionalproperties. If the designer is desirous of more isotropic properties inthe thermoplastic composite, any number of layers of this materialhaving fibers in various directions may be combined under heat andpressure to create a more isotropic material. Most often multiples of 4layers are used with −45°, 0°, 45°, 90° fiber orientations to producewhat are termed “quasi-isotropic” material properties.

Advanced Materials engineers finally discovered how to manufacture morecomplex thermoplastic composites with difficult plastics. The initialprocess consisted of adding chemical solvents to the heatedthermoplastic resin, to make the resin sufficient watery to saturate amass of fibers. Although this solvent had to be removed later in theprocess, composite manufacturers could now produce a thermoplasticcomposite with a relatively even distribution of plastic matrix to holdthe fibers in place.

Such methods of producing short fiber reinforced thermoplastics asdescribed above are widely practiced today with virtually allthermoplastics, but not with UHMW PE. However, is UHMW PE has a linearpolyethylene structure just like high density polyethylene but itsmolecules are exceptionally large, having a molecular weight of from 2to 6 million daltons. This high molecular weight provides UHMW PE withexceptional impact strength and abrasion resistance; however, it alsoresults in special well known processing characteristics which precludeuse of standard extrusion and molding techniques. In short, UHMW PE doesnot “flow” at high temperature as do virtually all other thermoplastics.In fact, when attempting to injection mold UHMW PE, the extremepressures result in shear-degradation of the polymer.

Because of UHMW PE's unusual behavior, the processes used to make UHMWPE stock materials (plate, sheet and bar being the most common forms ofUHMW PE) are most akin to compression molding or sintering. The typicalprocedure is to fill a form or mold with UHMW PE in powder form and thenapply heat and pressure to remove the air between the particles and toforce the particles to soften and deform and fuse into a single mass.This ideally results in an essentially void free solid althoughmicroscopic porosity will still exist. In some special cases a lowerpressure is used specifically to produce a porous structure. This poroussolid form of UHMW PE is sometimes used for filtration. These processesdo not require the UHMW PE to flow.

Desiring reinforced products of UHMW PE, many individuals have soughtways to successfully add short fibers to UHMW PE. U.S. Pat. Nos.4,055,862, 5,622,767 and 5,620,770 disclose successful methods formaking such “FRP” materials with UHMW PE and carbon fibers withoutrequiring the UHMW PE or the fibers to flow wherein the carbon fibersmay be up to 8 mm long. These patents disclose methods that areessentially the same as the method described above for makingunreinforced UHMW PE materials. In these “compression molding like” or“sintering like” methods the short carbon fibers are mechanically mixedwith the particles of UHMW PE powder prior to introducing the mixture tothe form or mold, and prior to melting the UHMW PE.

SUMMARY OF THE INVENTION

In one embodiment of the invention, the invention relates to compositematerial of continuous fibers and ultra high molecular weightpolyethylene, the composite material comprising ultra high molecularweight polyethylene, and continuous high strength fibers, wherein theultra high molecular weight polyethylene forms a continuous matrix amongand surrounding the fibers.

In another embodiment of the invention, the invention relates to acomposite material of continuous fibers and ultra high molecular weightpolyethylene, the composite material comprising ultra high molecularweight polyethylene; continuous high strength fibers, wherein the ultrahigh molecular weight polyethylene forms a continuous matrix among andsurrounding the fibers; and at least one additive or filler.

The invention also relates to a method of manufacturing an ultra highmolecular weight polyethylene composite, the method comprising selectingunidirectional and continuous high strength fibers; impregnating thefibers with ultra high molecular weight polyethylene in a fine powder toform a composite; optionally adding additives or fibers to thecomposite; and forming a continuous matrix of the ultra high molecularweight polyethylene surrounding the fibers.

The techniques described above, however, until the present invention,did not particularly work well with UHMW plastics. It is an object ofthe invention to provide a composite material of continuous fiber andultra high molecular weight polyethylene (UHMW PE) exhibits manyimproved physical properties in comparison to both unreinforced andshort fiber reinforced UHMW PE. Another object of the invention is toimprove strength and stiffness in the fiber direction as well as thermaland electrical conductivity when a highly conductive fiber is employed.It is yet another object of the invention to provide a composite thatexhibits many of the outstanding characteristics of unreinforced UHMWPE, such as excellent abrasion resistance, low coefficient of frictionand high PV.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a data table listing the properties of the prior artcomposites, as presented by Victrex USA Inc.

FIG. 2 is a data table listing the properties of the prior artunreinforced and reinforced composites, as manufactured by CytecEngineering Materials.

FIG. 3 is a data table listing the properties of the prior artunreinforced composite, as manufactured by Ticona Engineering Polymers.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention, which may be embodied in variousforms. Therefore, specific structural and functional details disclosedherein are not to be interpreted as limiting, but merely as a basis forthe claims and as a representative basis for teaching one skilled in theart to variously employ the present invention in virtually anyappropriately detailed structure.

Production of a continuous fiber UHMW PE composite is expected toexhibit very high strength and stiffness, excellent toughness,exceptional abrasion resistance, excellent chemical resistance, a lowcoefficient of friction and high bearing (PV) capacity. Continuousfibers are well known in the art as fibers extending to a length ofseveral feet or meters that maintain its structural strengthcontinuously throughout the fiber when the fiber is stretched. Dependingon the fiber chosen, the composite could also be highly conductive orhighly resistive, both electrically and thermally, or able to absorbgreat amounts of energy at high speed impact. To be successful infashioning such a composite, one would have to overcome the samepeculiar properties of UHMW PE that render the extrusion and injectionmolding of both unreinforced and short fiber reinforced UHMW PEproblematic.

In particular, all known methods of making continuous fiberthermoplastics require flow under pressure such as through film stackingmethods as practiced by Ten Cate and Bond Laminates or pultrusion typemethods practiced by others, including the pultrusion type methoddisclosed in U.S. Pat. No. 4,680,224.

In this invention, a slurry of UHMW PE powder, such as GUR 2126 made byTicona Engineering Polymers, a division of the Celanese Corporation,having a molecular weight of approximately 2,500,000 daltons, and wateris impregnated into a web of unidirectional fibers, such as AS4C carbonfibers made by Hexcel. FIG. 3 depicts selected physical properties ofunreinforced GUR 2126 UHMW PE, as published by Ticona. The webimpregnated with UHMW PE powder and water is then passed through a dryerto remove the water. The web impregnated with loose dry UHMW PE powderis heated and tensioned in such a way as to cause the UHMW PE to meltand fuse into a solid band under essentially no external pressuregreater than atmospheric pressure. The solid band then is then pulledthrough a heated die to reform it into the desired cross sectionalshape. This method requires no flow of the UHMW PE under high pressureand therefore the UHMW PE is not degraded by shearing.

The resultant continuous fiber UHMW PE composite produced using thismethod may be made in almost any cross sectional shape such as flatribbon, sheet, flat bar, round, square, triangle, channel, angle,I-beam, etc. The composite of the chosen cross sectional shape may thenbe cut into lengths or coiled for shipment and/or further fabricationdepending on its shape, thickness, stiffness, customer preference orother considerations.

In the process described above, the impregnation of the web of fiberswith a slurry of UHMW PE powder and water may be replaced byimpregnation of the web with dry UHMW PE powder in a fluidized bed.Using the fluidized bed, the dryer is unnecessary. The fluid in thefluidized bed may be any suitable gas, such as air.

Fabrication techniques well known to those schooled in the art ofcontinuous fiber thermoplastic composites may be used to make subsequentforms of the materials, parts or structures, such as slitting,pelletizing, weaving, laminating, tape placement, thermoforming, tablerolling, compression molding, bladder molding, machining, ultrasonicwelding, etc. for any number of end use products.

In other embodiments of the invention, the continuous unidirectionalfibers may be replaced by woven continuous fibers or randomly orientedcontinuous fibers, for example a felt or matted fabric of random fibers.

It should be noted that the strength and stiffness of the reinforcedUHMW PE in the fiber direction are dramatically higher than theunreinforced UHMW PE, as are the thermal and electrical conductivity.The PV (sliding bearing capacity) of the reinforced UHMW PE is alsosignificantly improved with the continuous fiber as is thermalexpansion. The composite maintains the low coefficient of friction andthe exceptional abrasion resistance of UHMW PE. The only loss ofproperty of the composite relative to the unreinforced UHMW PE is inimpact strength. This is typical of virtually all fiber reinforcedthermoplastics; however the impact strength of reinforced UHMW PE issignificantly higher than most other fiber reinforced thermoplastics.

It will be readily apparent to those skilled in the art that variouschanges and modifications of an obvious nature may be made, and all suchchanges and modifications are considered to fall within the scope of theappended claims. Other embodiments of the invention will be apparent tothose skilled in the art from consideration of the specification andpractice of the invention disclosed herein. It is intended that thespecification and examples be considered as exemplary only, with a truescope and spirit of the invention being indicated by the followingclaims and their equivalents.

1. A composite material of continuous fibers and ultra high molecularweight polyethylene, the composite material comprising: (a) ultra highmolecular weight polyethylene; and (b) continuous high strength fibers,wherein the ultra high molecular weight polyethylene forms a continuousmatrix among and surrounding the fibers.
 2. The composite material ofclaim 1, wherein the molecular weight of the ultra high molecular weightpolyethylene is greater than 2,000,000 daltons.
 3. The compositematerial of claim 1, wherein the high strength fibers comprise carbon,glass, aramid, boron, basalt, and steel.
 4. The composite material ofclaim 1, wherein the high strength fibers may be unidirectionalcontinuous fibers.
 5. The composite material of claim 1, wherein thehigh strength fibers may be woven continuous fibers.
 6. The compositematerial of claim 1,. wherein the high strength fibers may be randomlyoriented continuous fibers.
 7. A composite material of continuous fibersand ultra high molecular weight polyethylene, the composite materialcomprising: (a) ultra high molecular weight polyethylene; (b) continuoushigh strength fibers, wherein the ultra high molecular weightpolyethylene forms a continuous matrix among and surrounding the fibers;and (c) at least one additive or filler.
 8. The composite material ofclaim 7, wherein the molecular weight of the ultra high molecular weightpolyethylene is greater than 2,000,000 daltons.
 9. The compositematerial of claim 7, wherein the high strength fibers comprise carbon,glass, aramid, boron, basalt, and steel.
 10. The composite material ofclaim 7, wherein the high strength fibers may be woven continuousfibers.
 11. The composite material of claim 7, wherein the high strengthfibers may be unidirectional continuous fibers.
 12. The compositematerial of claim 7, wherein the high strength fibers may be randomlyoriented continuous fibers.
 13. The composite material of claim 7,wherein the additive or filler comprises other polymers.
 14. Thecomposite material of claim 7, wherein the additive or filler comprisesheat stabilizers.
 15. A method of manufacturing an ultra high molecularweight polyethylene composite, the method comprising: (a) selectingunidirectional and continuous high strength fibers; (b) impregnating thefibers with ultra high molecular weight polyethylene in a fine powder toform a composite; (c) optionally adding additives or fibers to thecomposite; and (d) forming a continuous matrix of the ultra highmolecular weight polyethylene surrounding the fibers.
 16. The method ofclaim 15, wherein the molecular weight of the ultra high molecularweight polyethylene is greater than 2,000,000.
 17. The method of claim15, wherein the high strength fibers comprise carbon, glass, aramid,boron, basalt, and steel.
 18. The method of claim 15, wherein the highstrength fibers may be woven continuous fibers.
 19. The method of claim15, wherein the high strength fibers may be randomly oriented continuousfibers.
 20. The method of claim 15, wherein the additive or fillercomprises other polymers.
 21. The method of claim 15, wherein theadditive or filler comprises heat stabilizers.